The organic EL device is a self-emitting device, and has been actively studied for their brighter, superior viewability and ability to display clearer images compared with the liquid crystal device.
In 1987, C. W. Tang et al. at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic EL device with organic materials. These researchers laminated an electron-transporting phosphor and a hole-transporting organic material, and injected both the charges into the phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (refer to Patent Documents 1 and 2, for example).
To date, various improvements have been made for practical applications of the organic EL device. In order to realize high efficiency and durability, various roles are further subdivided to provide an electroluminescent device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode successively formed on a substrate (refer to Non-Patent Document 1, for example).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and use of phosphorescent materials has been investigated (refer to Non-Patent Document 2, for example).
The light emitting layer can also be fabricated by doping a charge-transporting compound, generally called a host material, with a phosphor or a phosphorescent material. As described in the foregoing Non-Patent Documents 1 and 2, selection of organic materials in an organic EL device greatly influences various device characteristics, including efficiency and durability.
In an organic EL device, the charges injected from both the electrodes recombine at the light emitting layer to cause emission. However, because the holes have greater mobility than the electrons, some of the holes pass through the light emitting layer, and lower efficiency. Accordingly, there is a need for an electron transport material that has high electron mobility.
Tris(8-hydroxyquinoline)aluminum (hereinafter, referred to simply as “Alq3”), a representative light-emitting material, has been commonly used as an electron transport material. However, because of the slow electron mobility and the work function of 5.6 eV, it cannot be said that this material has a sufficient hole blocking performance.
One way of preventing some of the holes from passing through the light emitting layer and improving the probability of charge recombination at the light emitting layer is to insert a hole blocking layer. To date, various hole blocking materials have been proposed, including, for example, triazole derivatives (refer to Patent Document 3, for example), bathocuproin (hereinafter, referred to simply as “BCP”), and a mixed ligand complex of aluminum [aluminum(III)bis(2-methyl-8-quinolinate)-4-phenylphenolate (hereinafter, referred to simply as “BAlq”)] (refer to Non-Patent Document 2, for example).
On the other hand, 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter, referred to simply as “TAZ”) has been proposed as an electron transport material having an excellent hole blocking property (refer to Patent Document 3, for example).
TAZ has a large work function of 6.6 eV and a high hole blocking capability, and has been used as the electron-transporting hole blocking layer laminated on the cathode side of the fluorescent layer or phosphorescent layer produced by methods such as vacuum vapor deposition and coating. TAZ has contributed to improve the efficiency of organic EL devices (refer to Non-Patent Document 3, for example).
A major problem of TAZ, however, is the poor electron transporting property, and the material needs to be combined with an electron transport material of higher electron transporting property for the production of an organic EL device (refer to Non-Patent Document 4, for example).
BCP also has a large work function of 6.7 eV and a high hole blocking capability. However, because of the low glass transition point (Tg) of 83° C., the material has poor thin film stability, and cannot be said as being sufficiently functional as a hole blocking layer. In phosphorescent devices, it has been proposed to extend the device life by using BAlq as a hole blocking layer. While the device life can be extended by this approach, it is not possible to efficiently confine the holes in the light emitting layer because BAlq has only a small work function of 5.8 eV. The efficiency is thus inferior to a device using BCP, and it cannot be said that BAlq is satisfactory.
These materials all lack sufficient film stability, and are insufficient in terms of blocking holes. In order to improve the device characteristics of organic EL devices, organic compounds are needed that excel in electron injection and transport performance and hole blocking capability, and that has high stability in the thin-film state.
Improved compounds having an anthracene ring structure and a benzimidazole ring structure have been proposed (refer to Patent Document 4, for example).
However, devices using such compounds for the electron injection layer and/or the electron transport layer are still insufficient, even though luminous efficiency or the like is improved. Further improvements are thus needed to lower driving voltage, and increase luminous efficiency, particularly power efficiency.